Assessment of electron density effects on dose calculation and optimisation accuracy for nasopharynx,for MRI only treatment planning

Computed tomography (CT) is the gold standard for radiotherapy simulation and treatment planning, providing spatial accuracy, bony anatomy definition and electron density information for dose calculations. Magnetic resonance imaging (MRI) has been introduced in radiotherapy to improve visualisation of anatomy for accurate target definition and contouring, however lacks electron density information required for dose calculations, with various methods used to overcome this. The aim of this work is to assess the impact on dose calculation accuracy and optimisation results of different approaches to determine electron density, as could be used in MRI only treatment planning for nasopharyngeal datasets with VMAT treatment plans. Volumetric modulated arc therapy (VMAT) plans were created for 10 retrospective head and neck (H&N) nasopharyngeal patients. The VMAT plans were generated on the gold standard dataset, the original CT scan. Data sets with no density correction (water equivalent) and two different sets of bulk density correction for bone/air/tissue applied separately were generated for these patients and the VMAT plans were recalculated for each case. Plans were also reoptimised on these data sets, and recalculated. Optimisation error was assessed through equivalent uniform dose (EUD) differences. Additionally, point dose comparison, dose volume histogram (DVH) analysis and gamma analysis of dose were used to assess dose calculation error. The dose calculation error on average was an increase in EUD whereas the optimisation error on average was a reduction in EUD compared to the original plan for all datasets aside from the bone only override dataset where bone was set to 1.61 g/cm³. For the optimisation error, the largest mean absolute error (MAE) was 1.88 Gy EUD for the PTV, and 2.21 Gy EUD for the brainstem, for the reoptimisation completed on the air only overridden dataset, and recalculated on the original. Bulk density corrections for bone and air provide dose calculations within 3% of the original treatment plans. Optimisation errors have the potential to be greater than dose calculation errors if incorrect density corrections are utilized. Electron density correction using a bulk density approach achieves dose calculation uncertainties within 3%, however more advanced approaches, such as a voxel based approach, may improve accuracy and should be considered.